Effect Idempotency Key Store

An execution guard that makes retried requests safe: every request carries an idempotency key, and the store makes the key the single author of execution. The in-flight slot is claimed with one atomic Ref.modify, so of two racing duplicates exactly one runs the effect while the other awaits a Deferred the winner completes, and a late retry after completion replays the stored result inside its TTL without re-executing. Failures propagate to every waiting duplicate and release the key, so a declined card can be retried deliberately while a double charge stays impossible. Solves the double charge on retry and the check-then-insert race. Pinned to effect 4.0.0-beta.98.